This invention is directed to negative-acting photoimageable compositions which are developable in alkaline aqueous solutions. The invention is particularly applicable to primary photoimaging resists, but is applicable, as well, to compositions that are hardenable so as to form solder masks and the like.
A variety of such photoimageable compositions are described throughout the patent literature. Essential components of the type of photoimageable compositions to which the present invention is directed include I) a binder polymer, II) photopolymerizable .alpha.,.beta.-ethylenically unsaturated compound(s), and III) a photoinitiator chemical system. The binder polymer I) has sufficient acid functionality, generally carboxylic acid functionality, that the binder polymer is soluble in dilute alkaline aqueous solution and thereby renders the photoimageable composition developable in such alkaline aqueous solutions. The photopolymerizeable compound(s) II) are monomers and/or short chain oligomers, a substantial portion of which have multiple .alpha.,.beta.-ethylenic unsaturated functionality.
The photoinitiator chemical system III) includes chemicals which generate free radicals upon exposure to actinic radiation. These free radicals propagate the polymerization of the .alpha.,.beta.-ethylenic unsaturated moieties of the photopolymerizeable compounds II). Herein, the photoinitiator system III) is deemed to include not only chemical compounds which generate free radicals, but catalysts or sensitizers which promote the free-radical initiated polymerization of the .alpha.,.beta.-ethylenic unsaturated moieties of the photopolymerizeable compounds II).
Printed circuit boards almost invariably have through-holes to establish connections with circuitry on opposite faces of the board. Photoresists are required to "tent" these through-holes during processing. With holes becoming larger on circuit boards, higher tenting strength is becoming increasingly important; thus greater flexibility of photoimageable compositions after development is required. Improved flexibility also contributes to improved cross hatch adhesion which allows for better compatibility with automated polyester support film removal systems used to separate a support film from the photoresist after exposure and before development. If the photoresist is brittle, these support film removal systems will cause chipping of the exposed areas of photoresist predominantly at the edges of the panel and subsequently, circuit line defects.
By replacing a portion of conventional photoreactive monomers (like ethoxylated trimethylolpropane triacrylate) with an isocyanuric, urethane-based oligomer, a significant improvement to tenting strength and flexibility was observed. However, even though the flexibility was noticeably better, the fine line adhesion was not improved and the oligomer was shown to be a major source of developer scumming.
Improved flexibility, fine line adhesion and lower developer scumming has been demonstrated when the isocyanuric, urethane-based oligomer is comprised of the product of a polyethoxymono(meth)acrylate and the isocyanurate trimer of hexamethylene diisocyanate, as described, for example, in U.S. Pat. No. 5,744,282. The use of a (meth)acrylate-functional urethane product formed from a mono- or polyalkoxymono(meth)acrylolyl ester, such that the (meth)acrylate functionality is separated from the urethane linkage by one or more flexible alkylene oxide groups, in UV-curable photoresists enhances the performance of such compositions over those made with urethane compounds based on the isocyanurate trimer of hexamethylene diisocyanate. Alternatively, urethane oligomers have been proposed that are formed from monoalkoxymono- or di-caprolactone(meth)acrylolyl esters, which adds a mono- or di-caprolactone chain extension between the monoalkoxy(meth)acrylate functionality and the urethane linkage. Present day commercial applications require further improvements to flexibility, fine line adhesion and developer scumming, while not interfering with the chemical resistance of the photoresist to processing solutions and its stripping ability after formation of the patterned copper circuit lines.
Herein, novel (meth)acrylate-functional urethane oligomers based on polyalkoxy/polylactone (meth)acrylolyl esters are incorporated as at least a portion of the photopolymerizeable component II). The (meth)acrylate-functional urethane oligomers of this invention are found to significantly improve flexibility and fine line adhesion of the photoresist and minimize developer scumming. Along with improving the aforesaid properties, it has been found that (meth)acrylate-functional urethane oligomers further enhance the chemical resistance of the exposed photoresist to processing solutions, such as developing, plating and etching solutions, and also its stripping ability in strong alkaline aqueous solutions.